Add a step-by-step guide to README.md to help adopters get started

[mathieucaroff]

Something along the line of:

# Install rust with rustup
$ rustc --version
rustc 1.50.0 (cb75ad5db 2021-02-10)

git clone https://github.com/heinzelotto/rust-callgraph

cd rust-callgraph

# |
# | I'm not sure about that last call
# V
cargo +nightly r -- path/to/the_code_you_want_to_process.rs --sysroot /usr/local/

Also, it would be interesting to know whether rust-callgraph can work for multiple rust files, or only one at a time.

[farooqkz]

What is status of this issue?